I'd also have the firing pin protrusion checked.
If that doesn't do it, (and even if it does), put in heavier (Wolff from Brownell's) striker spring. Springs left long term in cocked bolts tend to shorten/weaken over time. I never leave a firearm cocked when in storage. Dry fire before storing.
I just replaced what I think was the original spring in my Rem 725- it was getting pretty mushy.. and rusty. The new, heavier than factory Wolff spring is a half inch longer... and crisp!
But I'm guessing gunk... After cleaning, use Corrosion X, Eezox, dry graphite, or any other dry lubricant in the bolt. Wet ones get gummy, attract and hold dirt, and get thicker in cold weather.
I use Corrosion-X or Eezox almost exclusively as they not only clean and lub., but protect (some) from corrosion. Be sure to let parts dry thoroughly before reassembly.
